Cytokines are secreted proteins that regulate cellular growth and differentiation. These factors are especially important in regulating immune and inflammatory responses, regulating lymphoid development and differentiation. Cytokines also have critical functions in regulating immune homeostasis, tolerance, and memory. Not surprisingly, cytokines are critical in the pathogenesis of autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease (IBD) and psoriasis. Understanding the molecular basis of cytokine action provides important insights into the pathogenesis of immune-mediated disease and offers new therapeutic targets. The first step in cytokine signaling is activation of receptor associated kinases, known as Janus kinases, or Jaks. Following the activation of Jaks by cytokines, the next step in signaling is the activation of a family of transcription factors called Stats (signal transducers and activators of transcription). Stat5A and Stat5B are important for T and B cell development in contrast, other STATs, STAT3, STAT4 and STAT6 are important for the differentiation of CD4 helper T cells. That is, T cells orchestrate immune responses through their production of cytokines, shaping the character of the response during host defense and in autoimmune settings. Subsets of helper T (Th) cells include Th1 Th2 cells and Th17 cells and . These subsets produce interferon gamma (IFNg), interleukin (IL-4) and IL-17. CD4+ T cells also become regulatory T (Treg) cells, which are essential for limiting immune responses. We found that deficiency of Stat5A/B in CD4+ T cells abrogated Treg development in vivo and blocked conversion of peripheral cells to become iTregs. To more carefully dissect the function of Stat5A/B in the immune system, we selectively deleted these related transcription factors CD4+ T cells to understand their role in T cell differentiation. In related studies, we also found that STAT5A/B regulate granulopoiesis. IL-17 is a major inflammatory cytokine, which appears to contribute to the pathogenesis of many autoimmune and autoinflammatory disorders including rheumatoid arthritis, spondyloarthropathies, multiple sclerosis and inflammatory bowel diseases (IBD). Of interest is that Th17 cells are thought to be developmentally related to regulatory T cells (Tregs) as both subsets can be induced from naive CD4+ T cells in the presence of transforming growth factor-beta (TGFb-1) in the context of different cytokines. We showed that Stat3 is essential for Th17 differentiation. We further showed using chromatin immunoprecipitation that Stat3 binds to the Il17 gene. Additionally, through analysis of patients with hyperimmunoglobulin E or Job's syndrome, who have dominant negative mutations of STAT3, we showed that Stat3 function is also essential for human Th17 differentiation. We extended our studies on the role of Stats in helper T cell differentiation by further investigating the role of Stat3 in IBD. In a T cell transfer model of colitis, we found that absence of Stat3 prevents the development of IBD. We also found that Stat3 was critical for proliferation and survival of CD4+ T cells. Using chromatin immunoprecipitation and massive parallel sequencing, we found that STAT3 bound many genes involved in Th17 differentiation, ranging from cytokines and cytokine receptors to transcription factors and genes that regulate cell proliferation and survival. In a collaborative study, we documented the critical need for IL-23 in the expansion and pathogenicity of Th17 cells. In effort to generate a mouse model of the cryopyrinopathy disorders, mice with mutatins of NLRP3 were created. Like patients with Neonatal Onset Multisystem Disease, Muckle-Wells and Familial Cold Autoinflammatory syndrome, mice with NLRP3 mutations have severe, overwhelmoing inflammatory disease. This disease is characterized by exagerated Th17 differentiation;however, the disease also occurs in the absence of adaptive immune cells. In a separate study, we also searched for sources of IL-17 and IL-22 in innate immune cells and found that lymphoid tissue inducer cells are important producers of these cytokines. We also sought to advance our understanding of Th1 and Th2 differentiation by defining the targets of STAT4 and STAT6, which regulate these process respectively. Again, we used Chip-Seq technology to define STAT4 and STAT6 binding sites on a genomewide scale. We related transcription factor binding with gene expression and epigenetic modification to define genes in which STATs were critical factors in these events. In another study, we collaborated to show that deletion of the kinase Lyn in myeloid cells resulted in enhanced Th2 differentiation that was driven by increased IL-4 production by basophils. This year, we have also initiated studies mapping genomewide chromatin modifications, examining the role of Stats in the process. Specifically, we analyzed histone modifications in Th1, Th2, Th17, and Treg cells by Chip-Seq technology. We have also assessed the importance of Stat4 in chromatin modifications in Th1 cells using Stat4-deficient mice and Chip-Seq technology.